The evolutionary origins of streptophyte multicellularity.

Over their more than 1 billion years of evolutionary history, streptophytes have repeatedly transitioned between unicellular, simple multicellular and complex multicellular forms. Rather than a linear trajectory toward increasing complexity culminating in land plants, streptophyte evolution is now understood as highly dynamic. Phylogenomic analyses reveal an early origin of multicellularity followed by lineage-specific losses, reductions and secondary gains of complexity. Filamentous and parenchymatous body plans in Charophyceae and Coleochaetophyceae are derived rather than direct precursors of land plant multicellularity, while zygnematophyte algae - the sister group to embryophytes - exhibit dramatic reductions to unicellular or filamentous forms despite retaining genetic toolkits linked to multicellular traits. We argue that streptophyte multicellular evolution is best explained by regulatory changes at the cellular level rather than gene gain or loss alone. Variation in cell division patterns, polarity establishment, cell wall sensing and intercellular connectivity plays a central role in shaping body plan diversity. We further propose that rewiring protein-protein interaction networks, mediated by intrinsically disordered regions and short linear motifs, represents a key mechanism driving repeated evolutionary innovation. Integrating comparative genomics, ancestral state reconstruction and functional cell biology will clarify how conserved molecular toolkits generated streptophyte diversity and ultimately facilitated the emergence of land plants.